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Fix inaccurate per-block metrics (#6491) 

`Measure` incorrectly assumed that it was the only thing being done by the parent `LoggingContext`. For instance, during a "renew group attestations" operation, hundreds of `outbound_request` calls could take place in parallel, all using the same `LoggingContext`. This would mean that any resources used during *any* of those calls would be reported against *all* of them, producing wildly inaccurate results.

Instead, we now give each `Measure` block its own `LoggingContext` (using the parent `LoggingContext` mechanism to ensure that the log lines look correct and that the metrics are ultimately propogated to the top level for reporting against requests/backgrond tasks).
This commit is contained in:
Richard van der Hoff 2019-12-09 11:55:30 +00:00 committed by GitHub
parent e519489fc4
commit 18660a34d8
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2 changed files with 19 additions and 42 deletions
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changelog.d/6491.bugfix Normal file
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@ -0,0 +1 @@
Fix inaccurate per-block Prometheus metrics.

60
synapse/util/metrics.py
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@ -91,72 +91,48 @@ class Measure(object):
__slots__ = [
"clock",
"name",
"start_context",
"_logging_context",
"start",
"created_context",
"start_usage",
]
def __init__(self, clock, name):
self.clock = clock
self.name = name
self.start_context = None
self._logging_context = None
self.start = None
self.created_context = False
def __enter__(self):
if self._logging_context:
raise RuntimeError("Measure() objects cannot be re-used")
self.start = self.clock.time()
self.start_context = LoggingContext.current_context()
if not self.start_context:
self.start_context = LoggingContext("Measure")
self.start_context.__enter__()
self.created_context = True
self.start_usage = self.start_context.get_resource_usage()
parent_context = LoggingContext.current_context()
self._logging_context = LoggingContext(
"Measure[%s]" % (self.name,), parent_context
)
self._logging_context.__enter__()
in_flight.register((self.name,), self._update_in_flight)
def __exit__(self, exc_type, exc_val, exc_tb):
if isinstance(exc_type, Exception) or not self.start_context:
return
in_flight.unregister((self.name,), self._update_in_flight)
if not self._logging_context:
raise RuntimeError("Measure() block exited without being entered")
duration = self.clock.time() - self.start
usage = self._logging_context.get_resource_usage()
block_counter.labels(self.name).inc()
block_timer.labels(self.name).inc(duration)
in_flight.unregister((self.name,), self._update_in_flight)
self._logging_context.__exit__(exc_type, exc_val, exc_tb)
context = LoggingContext.current_context()
if context != self.start_context:
logger.warning(
"Context has unexpectedly changed from '%s' to '%s'. (%r)",
self.start_context,
context,
self.name,
)
return
if not context:
logger.warning("Expected context. (%r)", self.name)
return
current = context.get_resource_usage()
usage = current - self.start_usage
try:
block_counter.labels(self.name).inc()
block_timer.labels(self.name).inc(duration)
block_ru_utime.labels(self.name).inc(usage.ru_utime)
block_ru_stime.labels(self.name).inc(usage.ru_stime)
block_db_txn_count.labels(self.name).inc(usage.db_txn_count)
block_db_txn_duration.labels(self.name).inc(usage.db_txn_duration_sec)
block_db_sched_duration.labels(self.name).inc(usage.db_sched_duration_sec)
except ValueError:
logger.warning(
"Failed to save metrics! OLD: %r, NEW: %r", self.start_usage, current
)
if self.created_context:
self.start_context.__exit__(exc_type, exc_val, exc_tb)
logger.warning("Failed to save metrics! Usage: %s", usage)
def _update_in_flight(self, metrics):
"""Gets called when processing in flight metrics